1. Field of the Invention
The present invention relates to a method for producing a composite oxide, and more particularly, relates to a method for producing a perovskite-type composite oxide which can be suitably used as a ceramic material for a ceramic electronic component.
2. Description of the Related Art
Barium titanate-based ceramics as one of perovskite-type composite oxides represented by the general formula ABO3 have been used widely as materials which have excellent dielectric characteristics in applications such as laminated ceramic capacitors.
Further, as a method for producing a composite oxide powder which has a perovskite structure represented by the general formula ABO3, for example, a method for producing a composite oxide powder has been proposed, which includes a mixing treatment step of mixing a hydroxide of an element constituting an A site component containing crystallization water with a titanium oxide powder which has a specific surface area of 250 m2/g or more, and in the method, the mixing treatment step includes a solution producing step of producing a solution with the A site component dissolved only with moisture of the crystallization water by carrying out a heat treatment, and a reaction step of reacting the titanium oxide powder with the solution to produce a reaction synthesis product, so that the solution producing step and the reaction step proceed continuously (claim 1 in Japanese Patent No. 4200427).
In addition, Japanese Patent No. 4200427 discloses the calcination of the obtained composite oxide (claim 7 in Japanese Patent No. 4200427).
Further, according to the inventive method in Japanese Patent No. 4200427, it is supposed that a composite oxide is achieved which has few heterogeneous phases, superfine grains, and excellent crystallinity, and the calcination treatment of the composite oxide can change the crystal system from the cubic composite oxide to produce a tetragonal composite oxide which is excellent in crystallinity.
In addition, as another method for producing a composite oxide, a method for producing a barium titanate powder has been proposed which includes a reaction step of adding a water-soluble barium compound to titanium oxide grains under an atmosphere in the absence of carbon dioxide at an equimolar ratio to the number of moles of the titanium oxide grains, and performing a reaction at a temperature of 100° C. or less in an aqueous solution at a pH of 11.5 or more and 13.0 or less (claim 2 in Japanese Patent No. 4057475).
According to the inventive method in Japanese Patent No. 4057475, it is supposed that a barium titanate powder which has a narrow grain size distribution can be produced economically while preventing the incorporation of chlorine impurities.
On the other hand, in laminated ceramic capacitors using a barium titanate (composite oxide)-based material as described above, the reduction in size and the increase in capacitance have been advanced, and such laminated capacitors have been put into practical use, in which dielectric layers are on the order of 1 μm in thickness and the number of dielectric layers stacked is greater than 800.
Further, in order to further reduce the laminated ceramic capacitors in size, there is a need to reduce the dielectric layers in thickness to a submicrometer area of 1 μm or less, and in order to achieve dielectric layers of submicrometer in thickness, there is a need to make the grain size of a ceramic sintered body constituting the dielectric layers fine down to 100 nm or less. To that end, an unsintered raw material powder (for example, a calcined powder of barium titanate-based material) itself for use in the formation of the dielectric layers is required to be small in grain size.
From the above point of view, while the inventive methods in Japanese Patent Nos. 4200427 and 4057475 meaningfully allow the achievement of a fine barium titanate powder, methods for efficiently producing a finer perovskite-type composite oxide with high reliability have been required actually in order to address further reduction in layer thickness in the future.